Compositions and processes for treating subterranean formations

ABSTRACT

A method and materials for stabilizing a wellbore against excess fluid pressure is described. It comprises forming or placing a flexible and essentially impermeable lining on or in the wellbore wall. The flexibility of the lining ensures that it remains in compression as the pressure in the wellbore is increased above the fluid pressure in the surrounding rock and it therefore does not need high tensile strength. The lining may be a preformed elastomer sleeve or formed in situ by the use of a reactive drilling fluid. Appropriate reactive formulations are described for the situation where the rock contains significant quantities of clay.

This invention relates to compositions and methods for stabilizingsubterranean formations surrounding a borehole. More specifically, itpertains to additives for drilling and remedial fluids or othermaterials and methods used to improve the mechanical properties of thewall of the borehole.

BACKGROUND OF THE INVENTION

Drilling operations typically involve mounting a drill bit on the lowerend of a drill pipe or “drill stem” and rotating the drill bit againstthe bottom of a hole to penetrate a formation, creating a borehole. Adrilling fluid, typically referred to as “drilling mud”, may becirculated down through the drill pipe, out the drill bit, and back upto the surface through the annulus between the drill pipe and theborehole wall. The drilling fluid has a number of purposes, includingcooling and lubricating the bit, carrying the cuttings from the hole tothe surface, and exerting a hydrostatic pressure against the boreholewall to prevent the flow of fluids from the surrounding formation intothe borehole.

A drilling fluid can place undesirable mechanical stress on the rockaround the wellbore and may even damage the reservoir. With increasingdepth a hydrostatic pressure acts outward on the borehole, which maycause mechanical damage to the formation and reduce the ability of thewell to produce oil or gas. Drilling fluids also may fracture theformation, requiring a drilling shut down in order to seal the fracture.Damage to a reservoir is particularly harmful if it occurs whiledrilling through the “payzone,” or the zone believed to hold recoverableoil or gas.

Therefore, after a section of the wellbore has been drilled, drillingoperations are stayed or ceased to seal the wellbore using a string ofpipe such as casing or a liner in the well bore. The stops are commonlyreferred to as “casing points”. At a casing point, a sealing compositionsuch as hydraulic cement slurry is pumped into the annular space betweenthe walls of the well bore and the exterior of the string of pipedisposed therein. The cement slurry is permitted to set in the annularspace thereby forming an annular sheath of hardened substantiallyimpermeable cement therein. The cement sheath physically supports andpositions the pipe in the well bore and bonds the pipe to the walls ofthe well bore whereby the undesirable migration of fluids between zonesor formations penetrated by the well bore is prevented. Thiswell-established technique has several disadvantages, including areduction in the well diameter after each casing point and the high costof the casing itself.

Thus, there is a continuing need for improved methods and sealingcompositions for sealing subterranean zones through which fluidsundesirably flow into or out of the wellbores penetrating the zones andfor simultaneously increasing the mechanical strengths of the wellbore.

PRIOR ART

In addition to the above-mentioned practice of casing and linercompletion, it is known to consolidate formations with a fluidcontaining polymerizable or hardening materials, such as epoxides,resins or isocyanates in combination with diols. Those methods aredescribed for example in the U.S. Pat. Nos. 6,012,524, 5,911,282,5,242,021, 5,201,612, 4,965,292, 4,761,099, 4,715,746, 4,703,800,4,137,971, or 3,941,191. In the recent International Patent ApplicationWO 99/31353 there are described various compositions to stabilize clayformations through in-situ polymerization that significantly changes theresistance of the clay to swelling and dispersion on contact with water.

It is further known to apply rubber and latex based materials forremedial operations, such as U.S. Pat. Nos. 4,649,998 and 5,159,980.These techniques usally assume the existence of drilled wellbore and areapplied locally to treat defects in the casing or cement. Or thetreatment is applied to weakly consolidated sand formations, i.e. inhighly porous formations.

In view of the above, it is an object of the invention to provide anovel method of stabilizing subterranean formations surrounding aborehole, particularly clayey or shale formations.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a flexiblelining to the wellbore. The lining material is designed to be moreflexible and less permeable than the formation so that when the wellborepressure exceeds the pore pressure the lining deforms more than the rockand remains in compression. The rock then provides mechanical support tothe lining material, which therefore does not need a high (tensile)strength.

The desirable property of the lining material is met when its shearmodulus G_(L) is smaller than the shear modulus of the formation rockG_(R). It is even more advantageous to have the ratio G_(L)/G_(R)limited by the Poisson's ratio ν of the lining.

In a preferred embodiment, the lining material has a reducedpermeability compared with the surrounding formation. Said formationbeing clayey implies that the lining is essentially impermeable.

According to a further preferred aspect of the invention, the lining isgenerated during or shortly after the drilling process. In other words,it is applied prior to casing and cementing and may extend the intervalbetween casing points.

In a more specific embodiment, the lining material is generated througha chemical reaction of a precursor material that, preferably, can beadded to the drilling fluid without impairing its other properties. Inanother specific embodiment the material is applied as a foil,preferably through an extrusion process or by pushing a liner from thesurface or a combination of both.

When the fluid pressure in the wellbore exceeds the pressure thatinitiates hydraulic fracturing in the unlined formation, fractures willbe initiated in the rock but they will be prevented from propagating bythe lining material which stops fluid from entering the fracture andpressurizing the crack tip. We also note that the rock may enterplasticity, so the tensile crack may not even be initiated. The resultof applying the lining will be an increase in the apparent fracturepressure of the rock formation, opening the mud window and allowing agreater length of well to be drilled before a conventional steel casingis set.

There are two main applications of the invention. Firstly, it isenvisaged that a drilling mud formulation containing a combination ofthe specified compounds described below may be used to maintain theintegrity of the wellbore during conventional drilling operations.Secondly, a fluid formulation containing a combination of the samecompounds may be used for general remedial operations in the wellbore.Finally, the invention may be used to achieve the goal of “casinglessdrilling”, that is to achieve with one and the same drilling andcompletion fluid the equivalent result of what is today obtained througha combination of drilling, casing and cementing operations. Or, theinvention may reduce the number or casing points required to completethe drilling.

These and other features of the invention, preferred embodiments andvariants thereof, and further advantages of the invention will becomeappreciated and understood by those skilled in the art from the detaileddescription following below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1. shows the resulting change in permeability as a reaction inaccordance with an example of the present invention progresses through acore sample;

FIG. 2. is a flow chart illustrating major steps of a method inaccordance with the present invention.

EXAMPLE(S) FOR CARRYING OUT THE INVENTION

The lining as envisaged by this invention could be either producedcontinuously during the drilling process or intermittently whiledrilling is suspended. Both preformed sleeving and material depositeddownhole to produce the lining are appropriate. An advantage of thepreformed sleeve is that the elastomer material properties can beoptimized without the constraints imposed by an in situ productionprocess. However, the lining formed downhole has the advantage ofconforming to the shape of the wellbore, which may be irregular.

The solid mechanics of an impermeable liner in a wellbore can besummarized for the case of a thin liner (compared to the radius of thewellbore) by a stress concentration factor K defined as: $\begin{matrix}{K = {- \frac{{G_{1}/G_{r}} - v}{1 - v}}} & \lbrack 1\rbrack\end{matrix}$where ν is Poisson's ratio for the lining and G_(l) and G_(r) are theshear modulus of the lining material and the rock respectively. Thelining is in tension if K is negative and the wellbore pressure exceedsthe far field pressure. Therefore, the lining will always be incompression when G_(l)/G_(r)<ν.

The structures of the chemicals compounds of potential use in theinventive process are shown in Table 1.

TABLE 1 Structures of Amine Aldehyde Chemicals. Compound STRUCTUREFormaldehyde

Glyoxal

Pyruvic Aldehyde

NAP

BNH2 CH₃CH(NH₂)CH₂—[OCH(CH₃)CH₂]_(l)—[OCH₂CH₂]_(m)—[OCH₂CH(CH₃)]_(n)—NH₂ 1,3 DA

1,2 DA

3NH2

DHB

Some trends can be discerned in the performance of the differentchemical structures. Considering first the amines, branched compoundsappear to perform better than the linear ones. Thus, 1,2 DA and 3NH2 aresuperior to 1,3DA and NAP respectively. Additionally, incorporatingEO/PO groups as with BNH2, which are known to adsorb strongly on theclay and inhibit clay swelling, did not result in impressivepermeability reductions. Comparing the aldehydes, the performance of thethree small aldehydes was similar, with pyruvic aldehyde marginallyexceeding the others.

The elasticity of the treated shale is a key parameter in the proposedwellbore lining technique, as the elastic modulus must be lower thanthat of the untreated shale for the lining to remain in compression asthe pressure in the wellbore increases.

The mechanical properties of films treated with formaldehyde, glyoxal orpyruvic aldehyde and either 1,2 DA or 3NH2 are shown in Table 2. The1,2DA pyruvic aldehyde treatment appears to increase both the tensilestrength and the Young's and shear modulus compared to the untreatedfilm. In all the other cases the treated films were more elastic thanthe untreated. Note that the 3NH2/pyruvic aldehyde has particularly goodperformance in that the tensile strength was increased in addition to asignificant the reduction in the Young's and shear moduli.

TABLE 2 Mechanical Properties of Treated and Untreated Clay FilmsTensile Young's Shear Clay Film Stress Modulus Modulus Treatment (MPa)(GPa) (GPa) Untreated samples 7.8 2.7 9.1 1,2DA/Glyoxal 12.2 2.2 7.31,2DA/Pyruvic 13.8 3.9 13.1 Aldehyde 3NH2/Formaldehyde 3.0 0.94 3.13NH2/Glyoxal 7.4 2.2 7.3 3NH2/Pyruvic 15.9 2.2 7.2 Aldehyde

In further experiments core samples were used to test the mechanicalproperties of shales modified in accordance with examples of theinvention. Shale samples were prepared by cutting 1 inch diameter coresinto 2 mm thick slices and polishing one face to a roughness of lessthan 1 μm using diamond paste. The prepared samples were tested in aconcentric ring-on-ring jig, which was built for this purpose.

Results from this test are shown in Table 3, below. All cores have beendrained at 10 MPa and exposed to fluids in a Hassler cell under aconfining pressure of 8.0 MPa.

The core which has been treated with a BNH2/Glyoxal solution was used inthe Hassler Cell experiment in which fluid was pumped through the corefor an extended period of time until no further change in permeabilityoccurred (see also FIG. 1 below). The Young's and shear modulus of thetreated Oxford Clay were smaller in both cases than those of theuntreated (pore fluid only) shale indicating the desired increase inflexibility, in contradiction of the results with the clay films. Whilethe BNH2/Glyoxal did little to the tensile strength of the core whiledrastically reducing the moduli, the 3NH2 increased the tensile strengthwhile marginally reducing the moduli compared to the core which had onlyseen pore fluid.

TABLE 1 Mechanical Properties of Treated and Untreated Shales Young'sShear Strength Modulus Modulus Shale Sample (MPa) (GPa) (GPa) Pore FluidOnly 5.3 6.51 19.7 BNH2/Glyoxal 5.2 0.27 0.82 3NH2/Pyruvic Aldehyde 7.95.75 17.4

Another aspect of the invention is the desired reduction in permeabilityafforded to the formation by the invention.

Experiments using a test cell with a thin clay film membrane show thatthe inherently low permeability of shale can be further reduced by avariety of chemical compositions. Results of those tests are shown inTable 4.

TABLE 4 Permeability Reduction in Clay Films Due to PolymerisationChemistry Permeability Reduced to Glyoxal/NAP 75% Glyoxal/BNH2 46%Glyoxal/1,3DA 55% Glyoxal/3NH2 42% Glyoxal/1,2DA 19% Formaldehyde/BNH266% Formaldehyde/3NH2 15% Pyruvic Aldehyde/3NH2 10% Pyruvic Acid/3NH236% DHB/3NH2 233

In addition to the clay membrane cell experiments, two Hassler Celltests were carried out on the 3NH2/Pyruvic aldehyde chemistry to confirmthat the behaviour observed in the thin films could be reproduced in ashale, Oxford clay. The experiments consisted of confining a core of theshale in a rubber sleeve at a pressure of 8.0 MPa while fluid was pumpedthrough the core at an inlet pressure of 7.5 MPa. The flow rate,normalised to the initial rate during flow of a synthetic pore fluid, isplotted for two concentrations of the pyruvic aldehyde FIG. 1. In bothHassler cell tests the concentration of 3NH2 was 5%.

The permeability has been reduced to 20% of the initial value, measuredduring flow of the synthetic pore fluid. Permeability reduction wasconsiderably slower for the 2% pyruvic aldehyde which levelled off ataround 40% of the initial value. These results are reasonably consistentwith the clay membrane cells with the same reactants in which thepermeability of the film was reduced to 60% and 10% for the low and highconcentration of the aldehyde respectively. The Hassler cell with the 5%aldehyde shows reduction in the permeability of 6%. The improvedperformance with an increase in aldehyde concentration indicates thatexperiments at still higher concentration should be carried out.

A wellbore simulator (SWBS) has been used to demonstrate that theconcept of the present is valid. This apparatus allows a rock core, inwhich a wellbore has already been drilled, to be exposed to a fluidunder realistic downhole conditions. The rock used in the experimentswas Oxford Clay. Cores 8 inches in diameter, 8 inches high and with awellbore 1 inch in diameter along the core axis were prepared by aninitial drainage period of 5 days at 10 MPa during which pore fluid wassqueezed out of the core. The drained core was then placed in thewellbore simulator and its fracture pressure measured.

The overburden, confining and mud pressures can be controlledindependently during an experiment. Firstly, the three pressures werestepped up to around 10 MPa while keeping any difference between thepressures small. Next the fluid filling the wellbore was circulated fora period of three days in the cases where the rock was exposed directlyto the fluid. This was to allow for the development of any chemicalinteraction between the rock and the fluid. Then the position of theoverburden piston was locked and the mud pressure was increased in stepsof 0.5 MPa while the confining pressure was maintained at a constantvalue of 10 MPa. The mud pressure was increased until a path opened upto allow the mud to communicate with the confining boundary, at whichpoint flow was observed out of the wellbore and into the confining fluidspace; usually coincident with a pressure spike in the confiningtransducer. Once this event occurred, the mud pressure was reduced tomatch the overburden and confining once more and then all three werereduced to ambient in step so that no further damage to the rock tookplace during depressurization. At the end of a test the rock was removedand dissected to establish the mode of failure and the invasion ofchemicals into the matrix.

Table 5 summarizes the results obtained for three mud systems and twoflexible lining techniques. The water based mud systems were formulatedwith Xanthan gum for viscosity control and FLRXL fluid loss control andthree different shale stabilizers: potassium chloride, potassiumchloride+sodium silicate and potassium chloride+sodiumsilicate+polyglycol. For all three mud systems the rock fractured atpressures in a narrow band between 15.6 and 16.5 MPa.

TABLE 5 Initial wellbore simulator results Failure Mud Lining MPaComment KCl/Polymer None 16.0– B154 16.5 Silicate None 15.7– B155 16.0Silicate + None 15.6– B153 Glycol 16.0 Water Silicone 19.2– B157 Rubber19.6 End Effect Water Rubber 21.1– B159 Sleve 21.7 O Ring Failed

The test with a silicone rubber lining was carried out as follows. Thecore was prepared by draining at 10 MPa as previously described,however, before it was placed in the wellbore simulator the wellbore andthe top and bottom surfaces of the core (but not the outer boundary)were coated with a layer of flowable silicone rubber (RS Components) toa thickness of 1 to 3 mm. The rubber was allowed to cure and then thecoated core was tested to fracture with water filling the wellbore. Thelining failed at a significantly increased pressure of between 19.2 and19.6 MPa. The failure was caused by an end effect as the ruptureoccurred where the lining passed between the rock and the overburdenpiston and the true increase in fracture pressure for an infinite,smooth wellbore would have been even higher.

In the final test the core was lined with a preformed tube offluorocarbon elastomer (Adpol, UK) of 0.8 mm thickness. The tube wassealed to the metal end platens that confine the top and bottom of thecore and then the fracturing test was carried out as described above.With this lining the pressure limit of one of the seals in the wellboresimulator piston failed at around 21.7 MPa, at which time the liningstill had not ruptured.

The last two test clearly demonstrate the large increases in fracturepressure possible by the application of a flexible, impermeable liningto the wellbore. The pressure at which the unsupported liners would haveburst can be estimated by assuming that they behave as an elastic hollowcylinder. The burst pressure, P_(b), is then given by $\begin{matrix}{P_{b} = {\frac{r_{e}^{2} - r_{i}^{2}}{r_{e}^{2} + r_{i}^{2}}\sigma_{t}}} & \lbrack 2\rbrack\end{matrix}$where σ_(t) is the tensile strength of the material and r_(e) and r_(i)are the external and internal radius of the cylinder respectively. Forthe silicone rubber, which had a tensile strength of 1.0 MPa, at auniform thickness of 2 mm the burst pressure would have been 0.2 MPawhen unsupported compared to the actual overbalance of 9.6 MPa atfailure. For the preformed elastomer tubing with a tensile strength of15 MPa the burst pressure would have been 1 MPa when unsupported by therock compared to the maximum measured overbalance of almost 12 MPa.

In FIG. 2, the main steps of the present invention are summarized asflow chart.

1. A method of stabilizing a wall of a wellbore penetrating asubterranean formation, said method comprising the steps of: (a)introducing a layer-forming material into said wellbore during thedrilling process; (b) letting said material form a layer supported bysaid wall; and (c) selecting said material such that the ratio of thelayer's shear modulus G_(l) over the formations shear modulus G_(r) issmaller than the Poisson's ratio ν of the layer material.
 2. The methodof claim 1, using the layer to reduce a stress concentration factor ofthe wall to less than zero.
 3. The method of claim 1, wherein thematerial forms the layer through a chemical reaction within thewellbore.
 4. The method of claim 3, wherein the material a mixture ofpyruvic aldehyde and a triamine.
 5. The method of claim 1, wherein thematerial is during a drilling operation continuously supplied from asurface location.
 6. The method of claim 1, wherein the first reactantis a diamine or a dihydric alcohol and the second reactant comprises atleast one carbonyl group.
 7. The method of claim 1, wherein the materialis selected such that the layer has a reduced permeability compared tothe formation permeability.
 8. A method of drilling a wellbore into apotentially hydrocarbon bearing formation comprising the steps of (a)drilling part of said wellbore (b) introducing during said drillingsteps a layer-forming material into said wellbore; (c) letting saidmaterial form a layer supported by said wall; and (d) selecting saidmaterial such that the ratio of the layer's shear modulus G_(l) over theformations shear modulus G_(r) is smaller than the Poisson's ratio ν ofthe layer material.